Method to improve matrix acidizing in carbonates

ABSTRACT

A process for improved acidizing in carbonate formations, where controlled pulse fracturing (CPF) is utilized in combination with a retarded acid and a solidifiable gel. The solidifiable gel forms a solid formation gel in a zone of greater permeability and a gel plug in the wellbore. Said gel is formed from a melamine formaldehyde resin and a cross-linkable polymer. An inhibited acid is placed in an interval of said formation having lessened permeability. The retarded acid comprises hydrochloric, formic, acetic acid or mixtures thereof which is placed in a wellbore adjacent the area to be treated. Said acid can contain retarders and corrosion inhibitors sufficient to make said acid stable in the wellbore. At least one CPF device is placed in said acid near the interval to be treated. Upon detonation of said device, acid is forced into the interval of lessened permeability, thereby enhancing the acidizing treatmemt.

RELATED APPLICATIONS

This application is a continuation-in-part of application Ser. No.036,742 which was filed on Apr. 10, 1987 now U.S. Pat. No. 4,787,456. Itis also related to Ser. No. 917,324 which was filed on Oct. 9, 1986 nowU.S. Pat. No. 4,834,180.

BACKGROUND OF THE INVENTION

It is a common practice to acidize subterranean formations in order toincrease the permeability thereof. For example, in the petroleumindustry, it is conventional to inject an acidizing fluid into a well inorder to increase the permeability of a surrounding hydrocarbon-bearingformation and thus facilitate the flow of hydrocarbonaceous fluids intothe well from the formation or the injection of fluids, such as gas orwater, from the well into the formation. Such acidizing techniques maybe carried out as "matrix acidizing" procedures or as "acid-fracturing"procedures.

In acid fracturing the acidizing fluid is disposed within the wellopposite the formation to be fractured. Thereafter, sufficient pressureis applied to the acidizing fluid to cause the formation to break downwith the resultant production of one or more fractures therein. Anincrease in permeability thus is effected by the fractures formed aswell as by the chemcial reaction of the acid within the formation.

In matrix acidizing, the acidizing fluid is passed into the formationfrom the well at a pressure below the breakdown pressure of theformation. In this case, increase in permeability is effected primarilyby the chemical reaction of the acid within the formation with little orno permeability increase being due to mechanical disruptions within theformation as in fracturing.

In yet another technique involving acidizing, the formation isfractured. Thereafter, an acidizing fluid is injected into the formationat fracturing pressures to extend the created fracture. The acidfunctions to dissolve formation materials forming the walls of thefracture, thus increasing the width and permeability thereof.

In most cases, acidizing procedures are carried out in calcareousformations such as dolomites, limestones, dolomitic sandstones, etc. Onedifficulty encountered in the acidizing of such a formation is presentedby the rapid reaction rate of the acidizing fluid with those portions ofthe formation with which it first comes into contact. This isparticularly serious in matrix acidizing procedures. As the acidizingfluid is forced from the well into the formation, the acid reactsrapidly with the calcareous material immediately adjacent to the well.Thus, the acid becomes spent before it penetrates into the formation asignificant distance from the well. For example, in matrix acidizing ofa limestone formation, it is common to achieve maximum penetration witha live acid to a depth of only a few inches to a foot from the face ofthe wellbore. This, of course, severely limits the increase inproductivity or injectivity of the well.

In order to increase the penetration depth, it has heretofore beenproposed to add a reaction inhibitor to the acidizing fluid. Forexample, in U.S. Pat. No. 3,233,672 issued to N. F. Carpenter, there isdisclosed an acidizing process in which inhibitor, such asalkyl-substituted carboximides and alkyl-substituted sulfoxides, isadded to the acidizing solution. Another technique for increasing thepenetration depth of an acidizing solution is that disclosed by U.S.Pat. No. 3,076,762 issued to W. R. Dill, wherein solid, liquid, orgaseous carbon dioxide is introduced into the formation in conjunctionwith the acidizing solution. The carbon dioxide acts as a coolant, thusretarding the reaction rate of the acid with the formation carbonates.Also, the carbon dioxide is said to become solubilized in the acidizingsolution, thus resulting in the production of carbonic acid whichchanges the equilibrium point of the acid-carbonate reaction toaccomplish a retarding effect.

An additional procedure disclosed in U.S. Pat. No. 2,850,098 issued toMoll et al. involves the removal of contaminants from a water well andthe adjacent formation through the injection of gaseous hydrogenchloride. Still another technique for acidizing a calcareous formationis disclosed in U.S. Pat. No. 3,354,957 issued to Every et al. In thisprocess liquid anhydrous hydrogen chloride is forced from a well intothe adjacent formations. The liquid hydrogen chloride vaporizes withinthe formation and the resulting gas dissolves in the formation water toform hydrochloric acid which then attacks the formation.

The effectiveness of acidizing in removing wellbore damage and improvingproductivity in carbonate reservoirs is highly dependent upon acidreactivity and contact with the formation in the vicinity of the damage.If the pay zone is extensive (greater than 20 to 25 feet in thickness),diverting methods, such as ball sealers, benzoic acid flakes or paraffinbeads, will be used to inject acid into the formation matrix over theentire interval. Where the total zone thickness is large (greater thanabout 25 feet), it is very difficult to effectively acidize the entireinterval, even when diverting agents are used.

Therefore, what is needed is a method whereby an interval in a formationcan be isolated, acidized and simultaneously fractured, wherein the acidin its reactive state can penetrate deeply into a formation, therebyincreasing its permeability.

SUMMARY OF THE INVENTION

This invention relates to a method for increasing the permeability of aformation which formation has varying zones of permeability where highenergy impulse fracturing is used in combination with an inhibited acid.In the practice of this invention, a pumpable solidifiable gel isdirected into a wellbore within the formation's zone of greaterpermeability under conditions to selectively close pores in said zone.The gel utilized is formed from a melamine formaldehyde resin and acrosslinkable polymer. Said gel is allowed to solidify in said zone ofgreater permeability and also in the wellbore where it forms a gel plugsufficient to support a desired volume of acid. Inhibited acid is placedabove said gel plug adjacent to a zone of lesser permeability.

A canister containing a propellant is suspended into said wellboresubstantially near said zone of lesser permeability. Later thepropellant is ignited and energy released sufficient to form more thantwo simultaneous radial fractures in said zone of lesser permeability.Release of this energy causes acid to enter said fractures and reacttherein. This reaction increases the permeability of said lesserpermeability zone.

It is therefore an object of this invention to create multiplesimultaneous radial fractures in a formation while acidizing saidformation.

It is another object of this invention to enhance the reactivity of anacid with the formation by contacting said acid with a greater area ofthe formation when multiple simultaneous radial fractures are created.

It is yet another object of this invention to increase the permeabilityof a formation and stimulate said formation to produce increased volumesof hydrocarbonaceous fluids.

It is still yet another object of this invention to increase thepermeability of a calcareous formation containing hydrocarbonaceousfluids for production therefrom while minimizing damage to the wellbore.

It is a still yet further object of this invention to provide a gelationreaction which can proceed under all pH conditions encountered in ahydrocarbonaceous reservoir.

It is even a yet further object of this invention to provide for asubstantially stable gel when high temperatures are encountered in areservoir.

It is even a still yet further object of this invention to provide for agelation reaction which will proceed in a saline hydrocarbonaceousreservoir environment.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic representation showing the placement of thecanister in the inhibited acid above the solidified gel plug prior toignition of the propellant.

FIG. 2 is a schematic representation depicting the creation ofsimultaneous fractures in a desired interval above the solidified gelplug.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the practice of this invention, referring to FIG. 1, a solidifiablegel material is directed into wellbore 10. This gel material is placedjust below interval 12 of formation 14 which is desired to be treated.Interval 12 contains a zone of lesser permeability. Interval 28 containsa zone of greater permeability. Said solidifiable material is of a sizesufficient to plug the more permeable zone 28 and said material ispumped into permeable zone 28 under conditions sufficient to selectivelyclose pores in more permeable zone 28. Said solidifiable material isallowed to set for a time sufficient to form a solid plug 16 sufficientto support an inhibited acid 18 thereabove while becoming a solid andclosing pores in interval 28 which has a zone of greater permeability.Thereafter, a canister 20 containing a propellant is placed intowellbore 10 via wire 24 above said solid gel plug 16 into inhibited acid18 and adjacent to interval 12 containing the zone of lesserpermeability. Afterwards, wellbore 10 is closed in a manner sufficientto withstand the ignition of a propellant contained in canister 20. Amethod for closing the well is disclosed in U.S. Pat. No. 4,039,030,which patent is hereby incorporated by reference.

Subsequently, the propellant contained in canister 20 is ignited. Uponignition, as shown in FIG. 2, the inhibited acid 18 is forced intointerval 12 containing said zone of lesser permeability 12 viasimultaneous multiple radial fractures 22 resultant from energy releasedfrom the propellant contained in canister 20 as a consequence of saidignition. Since interval 28 is closed by the solidified gel, thisinterval is not fractured or acidized. Once the inhibited acid is withinthe fractures, it becomes active and reacts with the walls of saidfractures, thereby increasing the size of said fractures. Said acid alsoincreases the distance the fractures penetrate into the formation. Thismethod is effective in acid treating an interval of a formation, wherethe total zone thickness is 25 feet or more. Acids which can be utilizedinclude hydrochloric acid, formic acid, acetic acid, gelled acids, andother similar acids known to those skilled in the art. When this methodis used, matrix acidizing of a formation is improved, particularlycarbonate containing formations.

As mentioned above, a pumpable gel mixture is directed into wellbore 10by pumping the gel mixture into said wellbore by pump means (not shown).After preferably from about 2 hours to about 4 hours, the pumpable gelmixture solidifies. As will be understood by those skilled in the art,the composition of the mixture can be varied to obtain the desiredrigidity in the gel. One method of making a suitable pumpable mixture isdisclosed in U.S. Pat. No. 4,333,461 issued to Muller on June 8, 1982.This patent is hereby incorporated by reference. The stability andrigidity of the solidified gel depends upon the physical and chemicalcharacteristics of the gel, which characteristics are selected so thatthe gel plug is of a stability and rigidity adapted to absorb the shockfrom ignition of the propellant. Generally, these pressures generatedupon ignition will vary from about 69,049 kPa to about 551,682 kPa(10,000 psig to 80,000 psig). Instantaneous heat generated upon ignitionof the propellant may be greater than about 538° C. (1,000° F.) in thevicinity of the deflagration but is quickly dissipated with propagation.

Often, depending upon the kind of propellant used, it will be necessaryto increase the density of the pumpable gel to obtain a gel plug havingthe desired stability and rigidity to absorb the generated energy. Toaccomplish this, any solid non-reacting material may be added to thepumpable gel mixture. Preferred non-reacting solid materials includesolid rock salt, or naturally occurring sodium chloride, calciumcarbonate, and suitably crushed mollusk shells, such as oyster shells.

Other gel mixtures can be used to obtain a solidified gel having thedesired stability and rigidity. A preferred gel mixture which can beused herein is formed from a melamine formaldehyde resin and acrosslinkable polymer.

The melamine formaldehyde ("MF") resin utilized herein is formed as areaction product of melamine and formaldehyde. Said resin is known as anaminoplast or amino resin which comprises a class of thermo-settingresins made by the reaction of an amine with an aldehyde. The resultantresin is reacted with a crosslinkable polymer in an aqueous medium underall pH conditions and needs no catalyst. Said polymer has at least onefunctional group selected from a member of the group consisting of anamine, an amide, a hydroxyl, or a thiol group. This reaction can becarried out at ambient conditions, and also under conditions occurringin a subterranean hydrocarbonaceous formation. The gel resultant fromsaid reaction can also be used in the recovery of hydrocarbonaceousfluids from a formation containing same.

These gels are novel in that they are unaffected by high salineconcentrations often encountered in oil reservoirs. High temperaturesencountered in said reservoirs do not adversely affect said gels.Carbonate, bicarbonate, and sulfate anions encountered in oil reservoirswhich are known to affect certain metal crosslinked gels do not affectthese novel gels. These novel gels can be formed under all pH conditionsand are particularly useful in pH conditions of 10 or less. A method formaking a kindred gel is discussed in U.S. Pat. No. 4,157,322 whichissued to Colegrove on June 5, 1979. Unlike Colegrove, the instantgelation reaction is not catalyzed by a salt which is acid generatingupon the application of heat. This patent is hereby incorporated byreference.

Polymers having functional groups such as NH₂, --CONH₂, --OH, --SH canbe gelled with methylated, MF resins. Some acceptable polymers includepolyacrylamide, Kelco's S-130 biopolymer, acrylamide modified polyvinylalcohol ("AMPVA"), Xanthan biopolymers, poly(acrylamide-co-acryl-amido-2-methyl-propanesulfonate) "AM-AMPS","Phillips HE" polymers (a family of acrylamide containing copolymers),and polyvinyl alcohol. More specifically, Phillips HE polymer comprisescopolymers of N-vinyl-2-pyrrolidone (VP) and acrylamide (Am) in whichthe weight ratios of VP:Am preferably range from about 30:70 to about70:30. Polymers mentioned in U.S. Pat. No. 4,157,322, supra, may beutilized as long as those polymers contain the functional groups abovementioned. Polymer concentration in said gels range from about 0.1 toabout 5.0 wt. percent. These polymer concentrations vary depending uponthe molecular weight of polymer used. Lower molecular weight polymersrequire a higher polymer concentration to gel. A polymer concentrationof about 0.2-5.0 wt. percent is preferred. This crosslinking/co-gelationmethod produces high integrity polymer gels able to withstand hightemperatures and high salinity conditions often found in subterraneanhydrocarbonaceous formations.

Methylated MF derived as a reaction product of melamine and formaldehydehas a molar ratio of between 1-6. A ratio of 3-6 is commonly found incommercial resins. The methoyl group, --CH₂ OH and its methylatedvarieties are reactive to various functional groups such as NH₂,--CONH₂, --OH, --SH and can also self-condense to form cured resins. Itspreparation is convenient and well documented in preparative polymermanuals.

The melamine resin that is utilized in this invention can be acommercial product such as Cyanamid's Parez® resins. Included amongthese melamine-formaldehyde (melamine) resins which are useful in thisinvention are the partially methylated resins and the hexamethoxymethylresins (i.e. American Cyanamid's Parez, Cymel™373, Cymel 370, Cymel 303,and Cymel 380). The resin, however, has to be one that is soluble ordispersible in an aqueous medium. Other amino resins can also be used.Non-limiting examples are urea-formaldehyde, ethylene and propylene ureaformaldehyde, triazone, uron, and glyoxal resins. The amount of MFresins required for adequate gel formation is in the ratio of 10:1-1:10polymer to amino resins. Preferred polymer concentrations are from about0.2 to about 5.0 wt. percent. Amino resins are preferred crosslinkersbecause they (1) are economical to use; (2) can be applied to a widevariety of polymers; (3) from thermally stable, brine tolerant gels; and(4) do not need an acid or base catalyst.

The gelation rate of the composition depends on the amount of each ofthe components and the temperature at which the reaction is conducted.Thus, one can tailor the gel rate and gel strength of the composition byadjusting the amount of the polymer, the resin amount and thetemperature. The higher the temperature at given concentrations of resinand polymer will result in a faster gelation time. If a thicker gelledcomposition is desired, the polymer and resin concentrations may beincreased for a given temperature.

Gels resultant from the gelation reaction were formed in about a 15 to30 wt.% brine solution containing at least about 1500 ppm Ca(II) and atleast about 500 ppm Mg(II). Said formed gels were stable as determinedby sustained gel integrity and low gel shrinkage for at least about 195°F. for at least three months. Examples of preferred gel compositions areset forth below.

    ______________________________________                                        Gelation of Melamine-Formaldehyde Crosslinker                                                        30%     Deionized                                                                             Parez                                  Example                                                                              Polymer         Brine.sup.8                                                                           Water   613.sup.1                              ______________________________________                                               10% AMPVA.sup.2                                                        #1     5 g             5 g     0         0.4 g                                #2     2.5 g           5 g     2.5  g    0.4 g                                       AMPS-AMPVA.sup.3 10%                                                   #3     2.5 g           5 g     2.5  g    0.4 g                                #4     5 g             5 g     0         0.4 g                                       PVA.sup.4 5%                                                           #5     5 g             2.5 g   2.5  g    0.4 g                                       AMPS-PVA.sup.5 10%                                                     #6     5 g             2.5 g   2.5  g    0.4 g                                       Magnifloc.sup.6 1%                                                     #7     5 g             5 g     0         0.4 g                                #8     5 g             2.5 g   2.5  g    0.4 g                                       AM-AMPS.sup.7 1%                                                       #9     5 g             5 g     0         0.4 g                                 #10   2.5 g           5 g     2.5  g    0.4 g                                ______________________________________                                        Gelation with trimethylolmelamine (TM)                                                               30%     Deionized                                      Example                                                                              Polymer         Brine.sup.8                                                                           Water   TM                                     ______________________________________                                               S-130 1%.sup.9                                                         #11    5 g             5 g     --        0.4 g                                #12    5 g             5 g               0.2 g                                       HE B 2%.sup.10                                                         #13    2.5 g           5 g     2.5  g    0.4 g                                #14    2.5 g           5 g     2.5  g    0.4 g                                       HE E 2%                                                                #15    2.5 g           5 g     2.5  g    0.4 g                                #16    2.5 g           5 g     2.5  g    0.2 g                                       Xanthan.sup.11 2%                                                      #17    2.5 g           5 g     2.5  g    0.4 g                                #18    2.5 g           5 g     2.5  g    0.2 g                                ______________________________________                                         .sup.1 A commercial 80% active amino resin obtainable from American           Cyanamid                                                                      .sup.2 Acrylamide modified polyvinyl alcohol                                  .sup.3 Acrylamido2-methyl-propanesulfonate/acrylamide modified polyvinyl      alcohol                                                                       .sup.4 Polyvinyl alcohol                                                      .sup.5 Acrylamido2-methyl propanesulfonate/polyvinyl alcohol                  .sup.6 Polyacryamide obtained from American Cyanamid                          .sup.7 Poly (acrylamideco-acrylamido-2-methyl-propanesulfonate)               .sup.8 30% NaCl, 2000 ppm Ca, 1000 ppm Mg                                     .sup.9 Kelco "S130" biopolymer                                                .sup.10 Phillips HE                                                           .sup.11 Pfizer Flocon biopolymer                                         

The gel plug as described herein is especially advantageous, since itwill not "melt" or break down because the intense heat generated by thedetonation of the high energy impulse device is localized. Once theacidizing method is completed, hydrochloric acid can be used tochemically break down any remaining gel plug or solid gel within themore permeable zone should it be desired to produce hydrocarbonaceousfluids from the acidized zone and the more permeable zones(s). Aconcentration of about 15% by weight HCl in water is especially useful.This acid with necessary corrosion inhibitors, etc., is available fromthe service companies. The HCl solution is circulated down the wellboreusing coiled tubing and nitrogen. It is often used with this operationto minimize the amount of fluid in contact with the zone of interestfollowing the high energy impulse treatment.

When using propellants to generate the desired fracturing pressure, itis often desirable to produce a gel plug which will withstandtemperatures from about 149° C. to about 232° C. (300° F. to 450° F.)for from about 0.5 of a day to about 4 days. A thermally stable solidgel plug is obtained by mixing into the pumpable gel mixture an oxygenscavenger chemical composition, for example, sodium thiosulfate or ashort-chain alcohol or carbinol (such as methanol, ethanol, andisopropanol). However, sodium thiosulfate is preferred. Theconcentration of the oxygen scavenger, of course, will depend upon thethermal stability desired to be obtained for the gel plug and thesolidified gel within said fractures. Moreover, the preferredconcentration of the oxygen scavenger chemical composition in thepumpable gel mixture is from about 0.10 percent by weight to about 0.75percent by weight, most preferably 0.50 percent by weight, based on thetotal weight of the mixture.

Several different ways are provided for easy removal of any remainingsolid gel plug. One variation, which can be utilized to facilitateremoval of the gel plug from the wellbore, is to form a solid gelcontaining a gel breaker. This gel breaker, included in the pumpable gelmixture, is selected from a group of chemical compounds which can breakdown the solid gel at temperatures of less than about 16° C. to about121° C. (60° F. to 250° F.). Generaly, this breakdown of the gel stemwill occur within from about 2 hours to about 24 hours aftersolidification of the gel mixture in the wellbore, depending upon typeand concentration of the gel breaker added. Chemical compositionssatisfactory for use as gel breakers, and which may be incorporated intothe pumpable gel mixture, include certain enzymes and certain oxidizingagents (such as sodium persulfate) which are suitable for breaking downthe solid gel. Other gel breakers are disclosed in U.S. Pat. No.4,265,311 issued to Ely on May 5, 1981. This patent is herebyincorporated by reference. Enzyme breakers may be obtained commerciallyfrom oil field service companies.

The concentration of the gel breaker incorporated into the pumpable gelmixture may vary from about 0.01 weight percent to about 0.10 weightpercent, preferably about 0.05 weight percent of the gel mixture.Although the temperature upon ignition in the wellbore may generallyexceed about 66° C. (150° F.), the gel plug will remain intact duringthe generation and dissipation of energy after ignition of thepropellant. Upon cooling to a temperature of from about 16° C. to 66° C.(60° F. to about 150° F.), a suitable gel breaker will break down thesolid gel plug stem in the wellbore, causing the plug to liquify.

Another method for breaking the gel according to the invention comprisescontacting the solidified gel stem with a mineral acid after ignition ofthe propellant and lapse of a suitable time interval. In those instanceswhere it is undesirable to have a gel breaker incorporated into the gelmixture prior to ignition to remove the solid gel plug, it is preferredto use hydrochloric acid of a strength sufficient to solubilize thesolid gel. Hydrocloric acid, and acids similar thereto, can be used tobreak down the solid gel on contact. Hydrochloric acid of aconcentration of about 5.0 percent to about 28 percent, preferably about15 percent by volume of solution, will generally be sufficient for thispurpose. Although hydrochloric acid has been mentioned, other similarmineral acids or strong organic acids may be used.

After the gel is solidified, a solution of inhibited acid is injectedinto the wellbore over the formed gel plug. Said inhibited acid does notdissolve the gel plug. The solution of acid employed may be any of theaqueous solutions of acid commonly employed for acidizing subterraneancalcareous formations. For example, the solution of acid may be anaqueous solution of hydrochloric acid. Commonly, the aqueous solutionsof hydrochloric acid employed for acidizing subterranean calcareousformations contain between 5 and 28 percent by weight of hydrogenchloride. An aqueous solution of acetic acid may be also employed.Additionally, an aqueous solution of formic acid may be employed. As isknown, when the acid solution becomes spent as the result of reactingwith the material of the formation, the solubility of calcium sulfate,i.e., anhydrite or gypsum, dissolved in the acid decreases. Thus, anycalcium sulfate dissolved from the formation or derived from the wateremployed in preparing the solution of acid can precipitate with aconsequent decrease in the permeability of the formation.

Accordingly, it is preferred that the solution of acid that is employedcontain an agent to inhibit the precipitation of calcium sulfate. Thus,where hydrogen chloride is employed, the solution thereof may contain upto 40 percent by weight of calcium chloride. Additionally, the solutionof acid may also contain any of the commonly employed inhibitors forpreventing corrosion of metal equipment, such as casing, liner, ortubing in the well.

The amount of solution of acid to be employed will vary according to theradial distance from the well to which the formation is to be acidizedand, as stated, this distance may vary up to 15 feet but will not, inmost cases, exceed about 10 feet from the well. The amount of solutionof acid to be employed will also vary according to the extent to whichthe material of the formation is to be dissolved. Preferably, the amountof acid should be one hydrocarbon pore volume of the portion of theformation to be acidized. However, lesser amounts may be employed.Generally, the amount employed will be that ordinarily employed toconventional, commercial acidizing operations.

Also, as disclosed in U.S. Pat. No. 3,233,672 issued to Carpenter,inhibitors, such as alkyl-substituted carboximides and alkyl-substitutedsulfoxides, can be added to the acidizing solution. This patent ishereby incorporated by reference.

After the inhibited acid has been placed into the wellbore to thedesired formation interval sought to be treated, a high energy impulsedevice or canister containing propellant therein is located within thewellbore. Propellant is contained in the canister or high energy devicesufficient to create simultaneous multiple radial fractures within theformation where the acid in its activated state reacts within saidfractures. Said fractures are enlarged and lengthened. Upon the creationof these fractures, the reactive acid is forced into said fractures.Once the acid has entered the formation via said fractures, the acidreacts with the formation, thereby increasing the permeability withinsaid formation. This increase in permeability allows for increasedvolumes of hydrocarbonaceous fluids to be produced from a formationcontaining same.

Said propellant can belong to the modified nitrocellulose or themodified or unmodified nitroamine propellant class. Another suitablepropellant is a composite propellant which contains ammonium perchloratein a rubberized binder. Other suitable propellants are discussed in U.S.Pat. No. 4,590,997 which issued to Stowe on May 27, 1986. This patent ishereby incorporated by reference.

After the steps of this method are completed, instead of removing thesolid gel plug, additional inhibited acid and another propellant devicecan be placed into the wellbore and the method repeated. Also, bydirecting additional solidifiable gel material into the wellbore, thelevel of the gel plug in the wellbore can be raised, and other desiredintervals can be treated where a multi-interval formation isencountered. If needed, the method can be repeated until the formationhas been acidized to the extent desired.

Although the present invention has been described with preferredembodiments, it is to be understood that modifications and variationsmay be restored to without departing from the spirit and scope of thisinvention, as those skilled in the art will readily understand. Suchmodifications and variations are considered to be within the purview andscope of the appended claims.

What is claimed is:
 1. A method for increasing the permeability of aformation which formation has at least one zone of lesser permeabilityand one zone of greater permeability where high energy impulsefracturing is used in combination with an inhibited acid comprising:(a)directing into a wellbore within said formation a pumpable solidifiablegel into a zone of the formation having a greater permeability underconditions to selectively close pores in said greater permeability zonewhere said gel comprises;(i) water; (ii) 0.2 to 5.0 wt. percent of across-linkable polymer having at least one functional group selectedfrom a member of the group consisting of an amine, an amide, a hydroxyl,or a thiol group; and (iii) 0.02 to 50.0 wt. percent of a partiallymethylated aminoplast resin which cross-links with said polymer; (b)allowing said pumpable gel to solidify and form a gel plug within saidwellbore which plug is sufficient to support a desired volume of acid;(c) placing an inhibited acid into said wellbore above said gel plugadjacent to said zone of lesser permeability; (d) suspending a canistercontaining a propellant into the wellbore substantially near said zoneof lesser permeability; and (e) igniting said propellant which releasesenergy sufficient to form more than two simultaneous radial fractures insaid zone of lesser permeability which also causes said acid to entersaid fractures and thereafter react within said fractures, therebyincreasing the permeability of said zone of lesser permeability.
 2. Themethod as recited in claim 1, where hydrocarbonaceous fluids areproduced from said formation after step (e).
 3. The method as recited inclaim 1, which comprises adding a solid non-reactant composition to thepumpable gel in an amount sufficient to increase the density of the gel,whereby within a time of up to about 4 hours said pumpable gel becomes asolid gel plug of sufficient density to withstand the energy releasedfrom said propellant when ignited, which ignition results in atemperature of up to greater than about 538° C. (1,000° F.) and pressureof about 551,682 kPa (80,000 psig).
 4. The method as recited in claim 1,wherein a solid is added to said pumpable gel and said solid is a memberof the group consisting of sodium chloride and calcium carbonate whichis added to said pumpable gel in an amount sufficient to increase thedensity of said gel, whereby within a time of up to about 4 hours saidgel becomes solid in said fractures while producing a gel plug ofsufficient density to withstand the energy released from said propellantwhen ignited, which ignition results in a temperature of up to greaterthan about 538° C. (1,000° F.) and a pressure of about 551,682 kPa(80,000 psig).
 5. The method as recited in claim 1, wherein a gelbreaker is added to the pumpable gel in an amount sufficient to breakdown the solid gel plug formed in the wellbore at a temperature of up toabout 121° C. (250° F.) within a time of up to about 24 hours.
 6. Themethod as recited in claim 1, wherein an oxygen scavenger is placed intosaid pumpable gel, and said oxygen scavenger is a member selected fromthe group consisting of sodium thiosulfate and a short-chain alcohol. 7.The method as recited in claim 1, wherein an oxygen scavenger isincluded in said pumpable gel in a concentration of from about 0.10percent by weight to about 0.75 percent by weight of said gel, and wheresaid oxygen scavenger is a member selected from the group consisting ofsodium thiosulfate and a short-chain alcohol.
 8. The method as recitedin claim 1, where said pumpable gel includes a gel breaker capable ofbreaking down said solid gel plug, and where said gel breaker comprisesan enzyme and/or an oxidizing agent.
 9. The method as recited in claim1, wherein hydrochloric acid in a concentration of from about 5 percentto about 28 percent by volume of solution is made to contact the solidgel and break down said solid gel plug to facilitate its removal fromthe wellbore.
 10. The method as recited in claim 1, where steps (a)through (e) are repeated and the height of solid gel plug is increasedso as to treat a different interval or zone in the formation.
 11. Themethod as recited in claim 1, where the zone or interval to be treatedis of a thickness greater than about 25 feet.
 12. The method as recitedin claim 1 wherein step (a) (iii) said resin is selected from a memberof the group consisting of melamine-formaldehyde, urea formaldehyde,ethylene urea formaldehyde, propylene urea formaldehyde, triazone, uronand glyoxal.
 13. The method as recited in claim 1 wherein step (a) (ii)said polymer is a member selected from the group consisting ofpolyacrylamide, polyvinyl alcohol, xanthan biopolymers, Kelco S-130biopoylmer, poly(acrylamide-co-acrylamido-2 methyl-propanesulfonate),Phillips HE polymers, and acrylamide modified polyvinyl alcohol.
 14. Themethod as recited in claim 1 wherein step (a) the ratio of polymer tosaid resin required for gelation is from about 10:1 to about 1:10.
 15. Amethod for increasing the permeability of a formation which formationhas at least one zone of lesser permeability and one zone of greaterpremeability where high energy impulse fracturing is used in combinationwith an inhibited acid comprising:(a) directing into a wellbore withinsaid formation a pumpable solidifiable gel into a zone of the formationhaving a greater permeability under conditions to selectively closepores in said greater permeability zone where said gel comprises;(i)water; (ii) 0.2 to 5.0 wt. percent of a cross-linkable polymer which isa member selected from the group consisting of polyacrylamide, polyvinylalcohol, xanthan biopolymers, heteropolysaccharide S-130, sodiumaligniate biopolymers, poly(acrylamide-co-acrylamido-2-methyl-propanesulfonate), Phillips HEacrylamide copolymer, and acrylamide modified polyvinyl alcohol havingat least one functional group selected from a member of the groupconsisting of an amine, an amide, a hydroxyl, or a thiol group; and(iii) 0.02 to 50.0 wt. percent of a partially methylated aminoplastresin which cross-links with said polymer thereby forming a gel of asize and strength sufficient to close pores in a zone of greaterpermeability in said formation; (b) allowing said pumpable gel tosolidify and form a gel plug within said wellbore which plug issufficient to support a desired volume of acid; (c) placing an inhibitedacid into said wellbore above said gel plug adjacent to said zone oflesser permeability; (d) suspending a canister containing a propellantinto the wellbore substantially near said zone of lesser permeability;and (e) igniting said propellant which releases energy sufficient toform more than two simultaneous radial fractures in said zone of lesserpermeability which also causes said acid to enter said fractures andthereafter react within said fractures, thereby increasing thepermeability of said zone of lesser permeability.
 16. The method asrecited in claim 15 where hydrocarbonaceous fluids are produced fromsaid formation after step (e).
 17. The method as recited in claim 15,which comprises adding a solid non-reactant composition to the pumpablegel in an amount sufficient to increase the density of the gel, wherebywithin a time of up to about 4 hours said pumpable gel becomes a solidgel plug of sufficient density to withstand the energy released fromsaid propellant when ignited, which ignition results in a temperature ofup to greater than about 538° C. (1,000° F.) and pressure of about551,682 kPa (80,000 psig).
 18. The method as recited in claim 15,wherein a solid is added to said pumpable gel and said solid is a memberof the group consisting of sodium chloride and calcium carbonate whichis added to said pumpable gel mixture in an amount sufficient toincrease the density of said gel, whereby within a time of up to about 4hours said gel becomes solid in said fractures while producing a gelplug of sufficient density to withstand the energy released from saidpropellant when ignited, which ignition results in a temperature of upto greater than about 538° C. (1,000° F.) and a pressure of about551,682 kPa (80,000 psig).
 19. The method as recited in claim 15,wherein a gel breaker is added to the pumpable gel in an amountsufficient to break down the solid gel plug formed in the wellbore at atemperature of up to about 121° C. (250° F.) within a time of up toabout 24 hours.
 20. The method as recited in claim 15, wherein an oxygenscavenger is placed into said pumpable gel, and said oxygen scavenger isa member selected from the group consisting of sodium thiosulfate and ashort-chain alcohol.
 21. The method as recited in claim 15, wherein anoxygen scavenger is included in said pumpable gel in a concentration offrom about 0.10 percent by weight to about 0.75 percent by weight ofsaid pumpable gel, and where said oxygen scavenger is a member selectedfrom the group consisting of sodium thiosulfate and a short-chainalcohol.
 22. The method as recited in claim 15, wherein said pumpablegel includes a gel breaker capable of breaking down said solid gel plug,and where said gel breaker comprises an enzyme and/or an oxidizingagent.
 23. The method as recited in claim 15, wherein hydrochloric acidin a concentration of from about 5 percent to about 28 percent by volumeof solution is made to contact the solid gel and break down said solidgel plug to facilitate its removal from the wellbore.
 24. The method asrecited in claim 15, where steps (a) through (e) are repeated and theheight of the solid gel plug is increased so as to treat a differentinterval or zone in the formation.
 25. The method as recited in claim15, where the zone or interval to be treated is of a thickness greaterthan about 25 feet.
 26. The composition recited in claim 15 wherein saidresin is selected from a member of the group consisting ofmelamine-formaldehyde, urea formaldehyde, ethylene urea formaldehyde,propylene urea formaldehyde, triazone, uron, and glyoxal.